The present invention relates to radiation curable compositions comprising at least one metallocene polyolefin. The radiation curable compositions are useful for a variety of applications, particularly as coatings and adhesives. The radiation curable composition may comprise a single metallocene polyolefin, or blend thereof. The ultraviolet curable compositions further comprise at least one photoinitiator and/or at least one photoinduced coupling agent. For pressure sensitive adhesive applications, the radiation curable composition also preferably comprises other ingredients such as a tackifying resins and plasticizers.
A continuing need in the coatings and adhesive art, particularly for pressure-sensitive adhesives (PSA""s), is the achievement of better control over various mechanical and process properties so that adhesives can be xe2x80x9ctailor-madexe2x80x9d for specific, highly demanding end-use applications such as packaging, medical, and masking tapes. These applications require a proper balance of properties, and this balance changes with the particular end-use. Similarly, laminating adhesives and coatings are needed which have low volatile content, immediate cure, and excellent adhesion to a variety of substrates, particularly film.
With the discovery of Ziegler-Natta catalysts, it became possible to polymerize xcex1-olefin monomers to high molecular weight. The homopolymers of the C6-C10 xcex1-olefins were naturally tacky and therefore good candidates for PSA""s since they had low toxicity, good aging, and favorable oxidative stability. These homopolymers are chemically inert, resistant to plasticizer migration, and relatively inexpensive. Such homopolymers, however, exhibit poor cohesive strength, lacking the shear adhesion necessary for high performance PSA""s.
There are several patents directed toward adhesive compositions comprising Ziegler-Natta catalyst derived polymerized xcex1-olefin monomers. Initially, such amorphous polyolefins were used alone or combined with other ingredients to produce hot melt adhesives. Since the amorphous polyolefins used for hot melt adhesives tended to exhibit diminished rather than enhanced shear strength upon exposure to radiation, new combinations of xcex1-olefin monomers that cross-link upon being exposed to radiation were developed.
For example, U.S. Pat. No. 5,112,882, issued to Babu, May 12, 1992; U.S. Pat. No. 5,209,971 issued to Babu, May 11, 1993, as well as WO 93/11184, published Jun. 10, 1993 and EP 0 620 247 A2 published Oct. 19, 1994, teach pressure sensitive adhesive compositions comprising one or more poly(xcex1-olefin) homopolymers, copolymers, terpolymers and tetrapolymers derived from monomers containing 6 to 10 carbon atoms and photoactive crosslinking agents; whereas U.S. Pat. No. 5,298,708, issued to Babu, Mar. 29, 1994, is directed toward a microwave-active tape employing the adhesive composition of the ""882 patent.
U.S. Pat. No., 5,202,361, issued to Zimmerman et al., Apr. 13, 1993, teaches radiation-curable, tackifier free compositions containing a blend of certain acrylate esters and monoethylenically-unsaturated copolymerizable monomers, certain xcex1-olefin polymers, and a photoinitiator.
U.S. Pat. No. 5,407,970, issued to Peterson et al., Apr. 18, 1995, teaches adhesive compositions comprising terpolymers C6 to C10 unsaturated xcex1-olefin monomers, C2 to C5 xcex1-olefin monomers, polyene monomers and an effective amount of photoactive cross-linking agent.
U.S. Pat. No. 5,559,164, issued to Babu, Sep. 24, 1996, teaches compositions radiation curable to pressure sensitive adhesives comprising one or more copolymers of xcex1-olefin of which 90.1 to 99 mole percent are one or more straight chain xcex1-olefins, having 2 to 10 carbon atoms of which at least 55% have 6 to 10 carbon atoms, and 9.9 to 0.1 mole percent are one or more xcex1-olefins having 6 to 20 carbon atoms and at least one methylidyne; an effective amount of a photoactive hydrogen abstracting agent; and optionally a tackifying resin.
xe2x80x9cMetallocene polyolefinsxe2x80x9d have recently been introduced. Metallocene polyolefins are homogeneous linear and substantially linear ethylene polymers prepared using single-site or metallocene catalysts.
Tse et al., U.S. Pat. No. 5,530,054, claims a hot melt adhesive composition consisting essentially of: (a) 30-70 wt-% of a copolymer of ethylene and about 6 to about 30 wt-% of a C4 to C20 xcex1-olefin produced in the presence of a catalyst composition comprising a metallocene and an alumoxane and having an Mw of from about 20,000 to about 100,000; and (b) a hydrocarbon tackifier which is selected from a recited list. Exemplified are compositions consisting of 45 wt-% of ethylene/butene-1 copolymer having a specific gravity of either 0.898 g/cm3 or 0.901 g/cm3.
Tse et al, U.S. Pat. No. 5,548,014, claims a hot melt adhesive composition comprising a blend of ethylene/xcex1-olefin copolymers wherein the first copolymer has a Mw from about 20,000 to about 39,000 and the second copolymer has a Mw from about 40,000 to about 100,000. Each of the hot melt adhesives exemplified comprises a blend of copolymers, contains 45 wt-% copolymer, with at least one of the copolymers having a polydispersity greater than 2.5. Furthermore, the lowest density copolymer exemplified has a specific gravity of 0.894 g/cm3.
Lakshmanan et al., U.S. Pat. No. 5,397,843, teaches blended polymer compositions comprising an admixture of a copolymer of ethylene and an xcex1-olefin and an amorphous polypropylene and/or amorphous polyolefin, or mixtures thereof. The single xcex1-olefin exemplified is xe2x80x9cFlexomer 9042xe2x80x9d from Union Carbide, having a 1-butene content of 15 wt-%, a melt index of 5.0 g/10 min., a crystallinity level of 26% and a density of 0.900 g/cm3. The xe2x80x9cFlexomerxe2x80x9d polyolefin depicted in the examples is believed to a have a polydispersity greater than 2.5.
Hence, metallocene polyolefins also found utility for use in finished hot melt adhesive compositions. Although such compositions are believed to be a substantial improvement over the amorphous polyolefins prepared using Ziegler-Natta catalysts, such compositions continue to suffer from poor shear adhesion strength.
There exists a need for adhesive compositions having improved shear strength that can be produced from commercially available materials. The present inventors have now discovered that adhesive compositions based on metallocene polyolefins may be radiation cured to enhance their adhesive properties, particularly shear strength. The resulting adhesive compositions are relatively inexpensive in comparison to the radiation curable compositions of the prior art, since it is not necessary for the adhesive manufacturer to first polymerize the xcex1-olefins. Further, the novel adhesive compositions are surmised to exhibit a balance of properties which is superior to those which have been previously attained.
The present invention describes metallocene polyolefin-based compositions that consistently crosslink to a reproducible extent upon exposure to a radiation source, thus substantially improving the properties of such compositions, such as the cohesive strength. Surprisingly, the metallocene polyolefin employed may have a significantly lower comonomer content relative to the xcex1-olefin based radiation curable compositions of the prior art. Further, the applicants have found that certain commercially available grades of metallocene polyolefins exhibit a significant increase in static shear upon exposure to radiation while concurrently retaining or improving the peel values.
Accordingly, the present invention is a radiation curable composition comprising at least one metallocene polyolefin. The radiation curable composition may comprise a single metallocene polyolefin or blends thereof. Preferably, the radiation curable composition comprises from about 15 wt-% to 100 wt-% of a metallocene polyolefin having a comonomer content ranging from about 10 mole-% to about 20 mole-%. The comonomer content of the metallocene polyolefin may also be expressed in terms of weight percent. Accordingly, the radiation curable composition of the present invention comprises at least one metallocene polyolefin having a comonomer content ranging from about 30 wt-% to about 50 wt-%.
Metallocene polyolefins having comonomer contents outside the range specified above may also be employed provided the comonomer content of the metallocene polyolefin blend meets the specified range.
In the case of ultraviolet (UV) curing, the radiation curable composition further comprises at least one photoinitiator and/or photoactive coupling agent.
The composition may have some degree of adhesive properties prior to curing which are enhanced upon exposure to a radiation source. However, in most instances the composition has poor adhesive and/or coating properties until after curing. In this instance, the composition is best described as an xe2x80x9cadhesive precursorxe2x80x9d or xe2x80x9ccoating precursorxe2x80x9d.
In one aspect, the present invention is a composition radiation-curable to a coating wherein the coating composition comprises at least one metallocene polyolefin. The radiation curable composition may be applied to various materials and substrates such as films and metals and cured to provide a coating with desired properties such as high cohesive strength, resistance to chemicals such as solvent, resistance to oxidative degradation, abrasion resistance, improved barrier properties, as well as combinations of such properties.
In another aspect, the present invention is a composition radiation curable to an adhesive. After curing the adhesive composition may be pressure-sensitive or alternatively relatively tack-free. The adhesive may further comprises additional ingredients such as tackifiers, plasticizers, additional polymers, and additional radiation responsive materials such as unsaturated polymers, oligomers, and monomers.
In another aspect, the present invention is a thermoplastic composition radiation curable to a film. In this embodiment, the composition may comprise additives commonly used in films, particularly slip agents. The composition may be coated onto a release liner or release coated belt, cured and stripped from the belt forming a type of cast film.
Additionally, the present invention relates to method of using a radiation curable composition comprising the steps of:
a) providing a composition comprising at least one metallocene polyolefin;
b) coating said composition onto a substrate; and
c) exposing said composition to an effective amount of radiation for curing.
Furthermore, the present invention also relates to a variety of articles such as medical tapes, masking tapes, pressure sensitive labels, laminates of polymeric film and foils, and industrial tapes comprising a substrate coated with a radiation cured composition, wherein said composition comprises at least one metallocene polyolefin.
Metallocene polyolefins are homogeneous linear and substantially linear ethylene polymers prepared using single-site or metallocene catalysts. Homogeneous ethylene polymers are characterized as having a narrow molecular weight distribution and a uniform short-chain branching distribution. In the case of substantially linear ethylene polymers, such homogeneous ethylene polymers are further characterized as having long chain branching. Substantially linear ethylene polymers are commercially available from The Dow Chemical Company as Affinity(trademark) polyolefin plastomers, which are produced using Dow""s Insite(trademark) technology. Homogeneous linear ethylene polymers are available from Exxon Chemical Company under the trade name Exact(copyright) plastomers.
The radiation curable composition of the present invention comprises at least one metallocene polyolefin such as homogeneous ethylene/xcex1-olefin interpolymers which are interpolymers of ethylene and at least one C3-C20 xcex1-olefins and particularly branched C3-C20 xcex1-olefins. The term xe2x80x9cinterpolymerxe2x80x9d is used herein to indicate a copolymer, or a terpolymer, or a higher order polymer. That is, at least one other comonomer is polymerized with ethylene to make the interpolymer.
The homogeneous ethylene/xcex1-olefin interpolymer is a homogeneous linear or substantially linear ethylene/xcex1-olefin interpolymer. By the term xe2x80x9chomogenousxe2x80x9d, it is meant that any comonomer is randomly distributed within a given interpolymer molecule and substantially all of the interpolymer molecules have the same ethylene/comonomer ratio within that interpolymer. The melting peak of homogeneous linear and substantially linear ethylene polymers, as obtained using differential scanning calorimetry, will broaden as the density decreases and/or as the number average molecular weight decreases. However, unlike heterogeneous polymers, when a homogeneous polymer has a melting peak greater than 115xc2x0 C. (such as is the case of polymers having a density greater than 0.940 g/cm3), it does not additionally have a distinct lower temperature melting peak.
In addition or in the alternative, the homogeneity of the polymer may be described by the SCBDI (Short Chain Branching Distribution Index) or CDBI (Composition Distribution Breadth Index), which are defined as the weight percent of the polymer molecules having a comonomer content within 50% of the median total molar comonomer content. The SCBDI of a polymer is readily calculated from data obtained from techniques known in the art, such as, for example, temperature rising elution fractionation (abbreviated herein as xe2x80x9cTREFxe2x80x9d), which is described, for example, in Wild et al., Journal of Polymer Science, Poly. Phys. Ed., Vol. 20, p. 441 (1982), in U.S. Pat. No. 4,798,081 (Hazlitt et al.), or in U.S. Pat. No. 5,089,321 (Chum et al.). The SCBDI or CDBI for the homogeneous ethylene/xcex1-olefin interpolymers useful in the invention are preferably greater than 50%, more preferably greater than 70%, with SCBDI""s and CDBI of greater than 90% being easily attained. It is important not to confuse the CDBI with the total weight percent of comonomer of the metallocene polyolefin. Although a certain weight percentage of molecules within a distribution have a comonomer content greater than 50% by weight, the metallocene polyolefin as a whole typically has a comonomer content of about 50 wt-% or less.
Homogeneous ethylene/xcex1-olefin interpolymers differ from amorphous polyolefins also described as amorphous polyalphaolefins (APAO), with regard to homogeneity, molecular weight distribution (Mw/Mn), as well as comonomer (xcex1-olefin) content. Amorphous polyolefins are homopolymers, copolymers, and terpolymers of C2-C8 xcex1-olefins which are typically polymerized by means of processes which employ Ziegler-Natta catalysts, resulting in a relatively broad molecular weight distribution, typically greater than 4. In contrast, the homogeneous ethylene/xcex1-olefin interpolymers useful in the inventive adhesive composition are characterized as having a narrow molecular weight distribution. The homogeneous ethylene/xcex1-olefins have a Mw/Mn of less than 4, prefeerably less than 3, more preferably from 1.5 to 2.5, even more preferably from 1.8 to 2.2, and most preferably about 2.0. Further, whereas amorphous polyolefins produced from Ziegler-Natta catalysts typically have an xcex1-olefin concentration greater than 50 wt-%, homogeneous ethylene/xcex1-olefin interpolymers useful in the present invention are predominantly ethylene, having a greater ethylene content than comonomer content.
Substantially linear ethylene interpolymers are homogeneous interpolymers having long chain branching. Due to the presence of such long chain branching, substantially linear ethylene interpolymers are further characterized as having a melt flow ratio (I10/I2) which may be varied independently of the polydispersity index, and the like, the molecular weight distribution Mw/Mn. This feature accords substantially linear ethylene polymers with a high degree of processability despite a narrow molecular weight distribution.
It is noted that substantially linear interpolymers useful in the invention differ from low density polyethylene prepared in a high pressure process. In one regard, whereas low density polyethylene is an ethylene homopolymer having a density of from 0.900 g/cm3 to 0.935 g/cm3, the homogeneous linear and substantially linear interpolymers useful in the invention require the presence of a comonomer which reduces the density to the range of from 0.855 g/cm3 to 0.910 g/cm3.
The metallocene polyolefins of the present invention contain pendent groups having readily reactive hydrogen atoms on tertiary carbon atoms and may contain residual unsaturation. When these polymers are exposed to radiation, such as ultraviolet (UV) in the presence of a photoinitiator and/or photoinduced coupling agent, or electron beam (EB), they crosslink leading to an improvement in the cohesive strength of the adhesive and/or coating. The metallocene polyolefin is polymerized from a comonomer having an effective amount of reactive hydrogen atoms on tertiary carbon atoms. Although, octene is the comonomer employed in the examples, better radiation responsiveness (i.e., greater increase in static shear values) is expected from metallocene polyolefins employing comonomers such as isoprene, and various branched comonomers such as 3-methyl pentene. A list of suitable comonomers that may be employed in the invention include methylidyne group-containing xcex1-olefins having 6 to 20 or more carbon atoms. Examples include, 4-methyl-1-pentene, 5-methyl-1-hexene, 4-methyl-1-hexene, ethenylcyclopentane, 6-methyl- 1-heptene, 5-methyl-1-heptene, 4-methyl-1-heptene, 4,5-dimethyl- 1-hexene,4-ethyl-1-hexene, ethenylcyclohexane, 7-methyl-1-octene, 8-methyl-1-nonene, 4,6-dimethyl-1-heptene, allylcyclohexane, 2-ethenylbicyclo[2.2.1]heptane (i.e., 2-ethenylnorborane, which has three methylidyne groups), 2-ethenyl-6-methylbicyclo[2.2.1]heptane (which has four methylidyne groups), 2-ethenyl 6-methylbicyclo[2.2.1]heptane (which has four methylidyne groups), 2-(3-propenyl)bicyclo[2.2.1]heptane (i.e., propenylnorborane, which has three methylidyne groups), 3-(3-propenyl)-2,6,6-trimethylbicyclo[3.1.1]heptane (i.e., 3-propenylpinane, which has four methylidyne groups), 1-ethenylpentacyclo[4.2.0.02,5.03,8.04,7] octane (i.e., ethenylcubane, which has seven methylidyne groups), 1-ethenyltricyclo[3.3.1.13,7] decane (i.e., ethenyladamantane, which has three methylidyne groups), and 1,2-dimethyl-5-ethenyltricyclo[3.3.1.13,7] decane (i.e., 1,2-dimethyl-5-ethenyladamantane, which has three methylidyne groups).
Other methylidyne group-containing xcex1-olefins that are suitable for use in the copolymers of the invention are the arylene, the catenary oxygen, and the catenary silicon group-containing xcex1-olefins, such as, for example, 4-(1-methyl-ethyl)-1-(3-propenyl)benzene, 4-(2-methylpropyl)- 1-(3-propenyl)benzene, 2-(1-methylethyl)4-pentyl-3-propenylnaphthalene, 3-(2-methylpropoxy)-1-propene,4-(2-methylpropoxy)-1-butene, 3-cyclohexoxy- 1-propene,dimethyl(1-methylethyl)-3-propenylsilane, bis(1-methylethyl)methyl-3-propenylsilane, and bis(1-methylethyl)methyl-4-butenylsilane.
A particularly preferred metallocene polyolefin contemplated for use in the invention comprises a homogeneous ethylene/xcex1-olefin interpolymer backbone that has been grafted with high density polyethylene or a synthetic wax in an amount ranging from about 0.1-15% by weight. The incorporation of the grafted polyethylene and/or wax segment results in an immediate improvement in cohesive strength and heat resistance even prior to curing. Exposing such compositions to radiation under the appropriate conditions induces crosslinking, which in turn, further enhances such properties.
The long chain branches of substantially linear ethylene interpolymers have the same comonomer distribution as the interpolymer backbone and can be as long as about the same length as the length of the interpolymer backbone. When a substantially linear ethylene/xcex1-olefin interpolymer is employed in the practice of the invention, such interpolymer will be characterized as having an interpolymer backbone substituted with from 0.01 to 3 long chain branches per 1000 carbons. Methods for determining the amount of long chain branching present, both qualitatively and quantitatively, are known in the art.
The molecular weight of the ethylene/xcex1-olefin interpolymer will be selected on the basis of the desired performance attributes of the radiation curable formulation. Typically, however, the ethylene/xcex1-olefin interpolymer will preferably have a number average molecular weight of at least 3,000, preferably at least 5,000. Typically, the ethylene/xcex1-olefin interpolymer will preferably have a number average molecular weight of no more than 200,000, more preferably no more than 100,000, and even more preferably less than 80,000.
Ultra-low molecular weight ethylene/xcex1-olefin interpolymers will be either ethylene homopolymers or interpolymers of ethylene and a C3-C20 xcex1-olefin. Such interpolymers are surmised to be particularly useful for room temperature applied and low application temperature ( less than 135xc2x0 C.) applied radiation curable compositions. When the ethylene/xcex1-olefin interpolymer has an ultra-low molecular weight, and the like, a number average molecular weight less than 11,000, the ethylene/xcex1-olefin interpolymer leads to a low polymer viscosity but is characterized by a peak crystallization temperature which is greater than that of corresponding higher molecular weight materials of the same density. The ultra-low molecular weight ethylene/xcex1-olefin interpolymers will have a number average molecular weight less than about 6000, preferably less than about 5000. Such homogeneous interpolymers will typically have a number average molecular weight of at least about 800, preferably at least about 1300.
The density of the metallocene polyolefin is a consequence of the type and amount of comonomer employed and will likewise be selected on the basis of the desired performance attributes of the radiation curable formulation. The metallocene polyolefin may be employed alone or compounded with additional ingredients such as waxes, plasticizers, and tackifiers. For metallocene polyolefins based on ethylene interpolymers copolymerized with comonomers such as octene, butene, and hexene, the density of the metallocene polyolefin ranges from about 0.850 g/cm3 to about 0.885 g/cm3. Typically, the metallocene polyolefin will have a density of no more than 0.880 g/cm3, preferably no more than 0.875 g/cm3, more preferably no more than 0.870 g/cm3.
The MI of the metallocene polyolefin employed in the composition of the present invention is typically greater than about 0.5 g/10 min., preferably greater than about 10 g/10 min., more preferably less than about 30 g/10 min.; and most preferred, particularly for low application temperature compositions, are metallocene polyolefins having a MI ranging from about 100 to about 5000 cps at 350xc2x0 F.
In the case of compounded compositions, the metallocene polyolefin will be present in the radiation curable compositions of the invention in an amount of at least 5 wt-%, preferably at least 10 wt-%, and more preferably at least about 15 wt-%. However, in other applications, such as EB curable coatings, the radiation curable composition may comprise up to 100 wt-% metallocene polyolefin.
The radiation curable composition may comprise a single homogeneous ethylene/xcex1-olefin interpolymer. In such an embodiment, the homogeneous ethylene/xcex1-olefin interpolymer will preferably have a density ranging from about 0.855 g/cm3 to about 0.890 g/cm3. When it is desired to prepare a radiation curable formulation with a minimal concentration of the homogeneous linear or substantially linear interpolymer, less than about 30 wt-%, the melt index (I2 at 190xc2x0 C.) of the homogeneous linear or substantially linear interpolymer will be preferably about 500 or less, more preferably about 30 or less, and most preferably less than about 10 g/10 min.
In the case of pressure sensitive adhesive radiation-curable compositions, preferred radiation curable compositions will comprise from about 10 wt-% to about 50 wt-%, preferably from about 20 wt-% to about 40 wt-% of a single homogeneous ethylene/xcex1-olefin interpolymer.
In another embodiment, a first homogeneous ethylene/xcex1-olefin interpolymer may be blended with a second homogeneous ethylene/xcex1-olefin interpolymer, wherein the first and second interpolymers differ in number average molecular weight by at least about 5000, preferably at least about 10,000, and more preferably at least about 20,000. In this embodiment, the combination of the lower molecular weight and higher molecular weight components will tend to yield an intermediate storage modulus at 25xc2x0 C. and an improved probe tack prior to curing.
In addition or in the alternative, a first homogeneous ethylene/xcex1-olefin interpolymer may be blended with a second homogeneous ethylene/xcex1-olefin interpolymer, wherein the first and second interpolymers differ in density by at least about 0.005 g/cm3, preferably by at least about 0.01 g/cm3. In this embodiment, particularly in the case of radiation curable pressure sensitive adhesives, as the density differential increases, the relative proportion of the higher density interpolymer will typically decrease, as the increased levels of crystallinity would otherwise tend to decrease storage modulus at 25xc2x0 C. and probe tack to levels which would render them unsuitable for use as pressure sensitive adhesive applications.
In one embodiment, the radiation curable composition will comprise a blend of two homogeneous ethylene/xcex1-olefin, the first interpolymer having a density of about 0.870 g/cm3 or less and the second interpolymer having density greater than about 0.900 g/cm3. In this instance, the density of the blend of metallocene polyolefins will continue to be less than about 0.885 g/cm3 as described previously. For lower viscosity adhesive precursor compositions, especially those which are radiation curable at temperatures less than about 163xc2x0 C. (325xc2x0 F.), the second homogeneous ethylene/xcex1-olefin interpolymer will have a greater density than the first homogeneous ethylene/xcex1-olefin interpolymer, and will preferably have a melt index greater than about 125, more preferably greater than about 500, and most preferably greater than about 1000 g/ 10 min.
In the case of UV curing, one or more photoactive initiators and/or photoactive coupling agents are added to the composition. Hydrogen abstracting type radical initiators are preferred. Representative examples include, but are not limited to aldehydes, such as benzaldehyde, acetaldehyde, and their substituted derivatives; ketones such as acetophenone, benzophenone, and their substituted derivatives (particularly the 4-alkylbenzophenones, wherein the alkyl group has 1 to 18 carbon atoms); quinones such as benzoquinone, anthraquinone, and their substitutes derivatives, thioxanthones, such as 2-isopropylthioxanthone and 2-dodecylthioxanthone; and certain chromophore-substituted halomethyl-sym-triazines, such as 2,4-bis(trichloromethyl)-6-(3xe2x80x2,4xe2x80x2-dimethoxyphenyl)-sym-triazine. Photoactive initiators that are preferred, since they are particularly effective in bringing about rapid gelation of the adhesive composition upon application of radiation, are polyfunctional benzophenones (i.e., compounds having aliphatic, aromatic, nitrogenous, silicic, or hetero atomic nuclei to which two to four benzoylphenoxy groups are attached). A particularly preferred photoactive hydrogen abstracting agent is polyfunctional derivatives of benzophenone containing at least one long chain aliphatic alcohol. A representative example is as follows: 
wherein Ph is phenol and Z is selected from the group consisting of aliphatic oligomers (i.e. ethylene-butylene), methyl, (C2nH2n+1), benzophenone, and mixtures thereof. It important to note that the above identified photoinduced couplings agents are novel in themselves and have utility in a variety of other radiation curable compositions (in the absence of metallocene polyolefins), having abstractable hydrogens such as polyester, polyacrylate, and other polyolefin based compositions.
Another suitable type of photoinitiator that may be employed in the compositions of the present invention is the xe2x80x9calpha cleavage typexe2x80x9d photoinitiator. This type is particularly beneficial when other unsaturated species such as acrylated oligomers and monomers are further employed. Alpha cleavage type photoinitiators are known in the art. Commercial examples include Irgacure 184 and Darocur 1173, both available from Ciba-Giegy (Hawthorne, N.Y.).
Photoactive initiators and photoinduced coupling agents are present at a concentration of from about 0.05 wt-% to about 3 wt-%, preferably from about 0.1 wt-% to about 2.0 wt-% and more preferably from about 0.5 wt-% to about 1.5 wt-% for UV curable compositions. However, in the case of electron-beam radiation, photoactive agents are not needed to crosslink the metallocene polyolefin.
To cure the composition of the present invention, a source of actinic radiation of sufficient energy (i.e., wavelength ranges) to generate free radicals when incident upon the particular photoinitiator selected for use in composition should be chosen. The preferred wavelength ranges for photoactive hydrogen abstracting agents disclosed above is 400 to 250 nm. The amount of radiant energy required to crosslink the adhesive film or coating of the present invention is 100 to 1500 mJ/cm2, more preferably 200 to 800 mJ/cm2, as measured with a Power-Puck(trademark) radiometer manufactured by EIT. Details of the photocure process are disclosed in U.S. Pat. Nos. 4,181,752 and 4,329,384.
Even in the absence of a photoactive hydrogen abstracting agent, the composition can be cured using electron-beam radiation. The dosage needed to crosslink the composition of the present invention varies depending on the particular composition but generally ranges from about 1 to 20 Mrads, preferably from about 2 to about 10 Mrads. Details of suitable processes for EB curing of adhesives-coated substrates can be found in U.S. Pat. No. 4,533,566, which is incorporated herein by reference.
In addition to or in the alternative, the radiation curable compositions of the present invention may further comprise a second solid or liquid radiation responsive material (RRM) that can polymerize or crosslink upon radiation exposure in an amount up to about 30 wt-%. Radiation responsive material contemplated for use in the invention include polyisoprene, preferably liquid; (meth) acrylated polyesters, (meth) acrylic monomers, (meth) acrylated block oligomers such as ethylenebutylene, (meth) acrylated polyolefins, (meth) acrylated urethanes, (meth) acrylated polyamides, and other oligomers and monomers. A further example of such is RRM 1, a radiation responsive material produced from reacting a hydroxy terminated ethylene-butylene liquid copolymer, such as Kraton(copyright) L-1203, available from Shell Chemical Company (Houston, Tex.), with an isocyanate terminated urethane acrylate oligomer at a 1:1.5 equivalence ratio to functionalize the OH group of the ethylenebutylene oligomer. As in the case of the above identified photoinduced coupling agent, RRM 1 is also novel in itself and has utility in a variety of other types of radiation curable compositions.
As used herein, the term xe2x80x9ctackifierxe2x80x9d means any of the compositions described below which are useful to impart tack to the hot melt adhesive composition. ASTM D-1878-61T defines tack as xe2x80x9cthe property of a material which enables it to form a bond of measurable strength immediately on contact with another surfacexe2x80x9d.
In general terms, tackifying resins are useful in the radiation curable composition of the invention. Tackifying resins comprise resins derived from renewable resources such as rosin derivatives including wood rosin, tall oil, gum rosin; rosin esters, natural and synthetic terpenes, and derivatives of such. Aliphatic, aromatic or mixed aliphatic-aromatic petroleum based tackifiers are also useful in the foams of this invention. Representative examples of useful hydrocarbon resins includes alpha-methyl styrene resins, branched and unbranched C5 resins, C9 resins, C10 resins, as well as styrenic and hydrogenated modifications of such. Tackifying resins range from being a liquid at 37xc2x0 C. to having a ring and ball softening point of about 135xc2x0 C. Solid tackifying resins with a softening point greater than about 100xc2x0 C., more preferably with a softening point greater than about 130xc2x0 C. are particularly useful to improve the cohesive strength of the adhesives of the present invention, particularly when only a single homogeneous ethylene/xcex1-olefin interpolymer is utilized.
For the radiation curable composition of the invention, the preferred tackifying resin is predominantly aliphatic. However, tackifying resins with increasing aromatic character are also useful, particularly when a second tackifier or mutually compatible plasticizer is employed.
The radiation curable composition of the invention may comprise from about 0 wt-% to about 70 wt-% of a tackifying resin. Typically, and particularly when it is desired to employ less than about 30 wt-% of the homogeneous ethylene/xcex1-olefin interpolymer, the radiation curable composition will comprise from about 20 wt-% to about 60 wt-%, more typically from about 30 wt-% to about 60 wt-% tackifier.
In the alternative, in cases where it is desirable to employ at least about 30 wt-% of the homogeneous ethylene/xcex1-olefin interpolymer, the present invention advantageously provides radiation curable formulations which contain minimal tackifier, and the like, less than about 30 wt-% tackifier, preferably less than about 25 wt-% tackifier, more preferably less than about 20 wt-% tackifier, and most preferably less than about 15 wt-% tackifier. In such instances, radiation curable compositions containing less than about 10 wt-% tackifier, and even compositions having no tackifier, exhibit adequate tack for adherence to a substrate prior to curing.
A plasticizer is broadly defined as a typically organic composition that can be added to thermoplastics, rubbers and other resins to improve extrudability, flexibility, workability, or stretchability. In preferred embodiments of the invention, a plasticizer will be present in the radiation curable composition in an amount up to about 60 wt-%, particularly for compositions having relatively low peel strength. In the case of radiation curable compositions having a high degree of pressure sensitivity, the plasticizer is preferably employed in an amount ranging from about 20 wt-% to about 40 wt-%. Higher amount of plasticizer may be employed in radiation curable xe2x80x9coil-gelsxe2x80x9d. The plasticizer may be either a liquid or a solid at ambient temperature. Exemplary liquid plasticizers include hydrocarbon oils, polybutene, liquid tackifying resins, and liquid elastomers, such as liquid polyisoprene. Plasticizer oils are primarily hydrocarbon oils which are low in aromatic content and which are paraffinic or napthenic in character. Plasticizer oils are preferably low in volatility, transparent and have as little color and odor as possible. The use of plasticizers in this invention also contemplates the use of olefin oligomers, low molecular weight polymers, vegetable oils and their derivatives and similar plasticizing liquids.
When a solid plasticizing agent is employed, it will preferably have a softening point above about 60xc2x0 C. It is believed that by combining the homogeneous ethylene/xcex1-olefin interpolymer with a suitable tackifying resin and a solid plasticizer such as a cyclohexane dimethanol dibenzoate plasticizer, the resulting radiation curable composition may be applied at temperatures below about 120xc2x0 C., preferably below about 100xc2x0 C. Although a 1,4-cyclohexane dimethanol dibenzoate compound commercially available from Velsicol under the trade name Benzoflex(trademark) 352 is exemplified, any solid plasticizer that will subsequently recrystallize in the compounded thermoplastic composition is suitable. Other solid plasticizers that may be suitable for this purpose are described in EP 0422 108 B1 and EP 0 410 412 B1, both assigned to H.B. Fuller Company.
Waxes may be usefully employed in the radiation curable compositions of the present invention, particularly when the composition is intended to be relatively tack free upon cooling and solidifying, such as the protective coatings. Waxes are commonly used to modify the viscosity and reduce tack at concentrations up to about 20 wt-%, preferably less than about 10 wt-%. Waxes useful in the radiation curable compositions of the present invention include paraffin waxes, microcrystalline waxes, Fischer-Tropsch, polyethylene and by-products of polyethylene wherein Mw is less than about 3000.
Also suitable are waxes prepared using a constrained geometry catalyst. Ultra-low molecular weight ethylene/xcex1-olefin interpolymers having a relatively high density, greater than about 0.920 g/cm3 may be referred to as homogeneous waxes and are set forth in the examples below. Homogeneous waxes, in contrast to paraffinic waxes and crystalline ethylene homopolymer or interpolymer waxes, will have a Mw/Mn of from about 1.5 to about 2.5, preferably from about 1.8 to about 2.2.
Homogeneous waxes lead to a low polymer and formulation viscosity, but are characterized by peak crystallization temperatures which are greater than the peak crystallization temperatures of corresponding higher molecular weight materials of the same density. In adhesive applications, the increase in peak crystallization temperature translates to an increased heat resistance, and the like, improved creep resistance in pressure sensitive adhesive foam, and improved shear adhesion failure temperatures (SAFT) in the uncured adhesive composition.
As is known in the art, various other components can be added to modify the tack, color, odor, etc., of the radiation curable thermoplastic composition. Additives such as antiblock additives, pigments, and fillers, can also be included in the formulations. It is generally preferred that the additives should be relatively inert and have negligible effects upon the properties contributed by the homogeneous linear or substantially linear interpolymer, tackifying agent, and plasticizer.
The metallocene polyolefins of the present invention can be produced in accordance with any known polymerization process, including slurry, or solution phase polymerization, gas phase polymerization, and high pressure polymerization process. Solution phase polymerization is surmised to most amenable to branched comonomers and those having relatively larger molecular structure. Alternatively, branched comonomers such as 3-methyl-pentene, 4-methyl-heptene and 4,5 dimethyl 1-hexene, as well as high density polyethylene or waxes may be grafted onto an ethylene backbone. Grafting can be achieved by feeding the metallocene polyolefins and the material to be grafted (i.e. high density polyethylene) in the presence of a suitable free radical generator such as peroxide into a melt extruder. More information pertaining to the characteristics and polymerization of grafted ethylene xcex1-olefin polymers, can be found in U.S. Pat. No. 5,126,199 and U.S. Pat. No. 5,185,199, incorporated herein by reference, both assigned to The Dow Chemical Company.
After polymerization of the metallocene polyolefins containing branched comonomers such as 3-methyl-pentene, 4-methyl-heptene and 4,5dimethyl 1-hexene, as well as high density polyethylene or wax, the polymer would then optionally be combined with tackifiers and plasticizers at the amounts previously specified. The resulting composition could then be coated onto Mylar(trademark) at a thickness of about 1 mil and exposed to a EB radiation source. Alternatively, a photoinitiator and/or photoinduced coupling agent may be further added and the resulting composition exposed to UV.
The radiation curable compositions of the invention may be prepared by standard melt blending procedures. In particular, the first polymer(s), tackifier(s), and optional plasticizer(s) may be melt blended at an elevated temperature (from 150xc2x0 C. to 200xc2x0 C.) under an inert gas blanket until a homogeneous mix is obtained. Any mixing method producing a homogeneous blend without degrading the hot melt components is satisfactory, such as through the use of a heated vessel equipped with a stirrer.
Further, the homogeneous ethylene/xcex1-olefin interpolymer(s), optional tackifier(s) and optional plasticizer(s) may be provided to an extrusion coater for application to the substrate.
When the ethylene/xcex1-olefin interpolymer is a blend of two or more ethylene/xcex1-olefin interpolymers, it will be preferred to prepare the radiation curable pressure sensitive adhesive compositions using a dual reactor configuration, with one of the polymers being produced in the first reactor, the other of the polymers being produced in a second reactor, and the tackifier(s) and optional plasticizer(s) being optionally provided, typically at a point after the second reactor, via a side-arm extruder. In this embodiment, radiation curable pressure sensitive compositions can be provided in forms such as pellets, pillows, or any other desired configuration. Examples of such a process which may be adapted in accordance with the teachings of this disclosure to prepare blends of a homogenous linear (higher molecular weight or ultra-low molecular weight) or substantially linear ethylene/xcex1-olefin interpolymer, wax, and optional tackifier, are disclosed in WO 94/00500 and WO 94/01052.
The radiation curable composition of the present invention can be coated from solution or applied molten by any of a variety of coating processes well known in the art including knife coating, roll coating, gravure coating, pattern coating, curtain coating, etc. Useful coating thickness"" range from about 12.5 to 500 xcexcm, preferably from about 25 to about 200 xcexcm, and more preferably from 25 to 150 xcexcm.
High viscosity metallocene based composition may be applied molten or cast from a solvent solution and cured with a variety of radiation sources, particularly ultraviolet or electron beam. The low viscosity metallocene based compositions, may be advantageously applied at low application temperature, ranging from room temperature to about 135xc2x0 C. In this instance, the radiation curable composition preferably has a viscosity less than about 50,000 cPs at 135xc2x0 C. (275xc2x0 F.), preferably less than about 30,000 cPs, and more preferably less than about 10,000 cPs.
The preferred adhesive compositions will have a 180xc2x0 peel of greater than about 1000 g/2.5 cm (2.2 pounds/linear inch), a loop-tack of greater than about 1200 g/2.5 cm2 (2.5 pounds/inch2) and a shear value at room temperature of greater than about 24 hours for a 1 kg load. More preferably, the adhesive compositions exhibit shear values greater than about 24 hours for a 1 kg load at elevated temperatures, 70xc2x0 C. Most preferably, the peel value is greater than about 2000 g/2.5 linear cm (4.0 pli) and the loop tack in excess of 2000 g/2.5 cm2 (4.5 psi).
A PSA tape can be made from the radiation curable composition of the present invention by coating onto a substrate a composition comprising at least one metallocene polyolefin and exposing the coated substrate to sufficient UV or EB radiation to effectively crosslink the composition to provide a PSA. The tapes may be employed for various application including medical tapes, masking tapes, pressure sensitive labels, laminates of polymeric film and foils, industrial tapes such as automotive tapes as well as highway marking tapes.
Crosslinked compositions derived from the composition of the present invention preferably have a gel content (when corrected for soluble tackifying resins and other additives) in the ranges of from 2 to 95 wt-%, more preferably from 30 to 80 wt-% and most preferably from 50 to 70 wt-%.
A wide range of materials can be used as substrates. Common examples include polyethylene terephthalate, hereinafter PET, various polyolefin and other plastic films such as polycarbonate and polyurethane, woven and nonwoven fabrics, metals and metal foils, paper, glass, ceramics, and composite materials comprised of laminates of one more of these materials. The radiation curable composition of the present invention can also be coated onto release liners, such as those described in U.S. Pat. Nos. 4,386,135; 3,957,724 and 2,532,011 and subsequently radiation cured to form adhesive transfer films.
In some applications, primers can be used to improve the adhesion of the composition to some substrates.
For coating applications, the metallocene based composition may be applied to various materials such as films and metals and cured to provide a coating with desired properties such as high cohesive strength, resistance to chemicals such as solvent, oxidative degradation resistance, abrasion resistance, improved moisture and gas barrier properties, as well as combination of such properties.